（a）It is wrong to tell a lie.
撒谎是不对的。
（b）He does not think it wrong to tell a lie.
他不认为撒谎是不对的。
【解说】
这种形式上的主语或宾语，是中文表现法中所没有的。这些句子的真正的主语或宾语移到后面去了，而不在原来的地位上，于是那里就产生一个空缺，只得用“if”填补上去，所以这个“it”，英文就叫做填补词（expletive）。
如果把（a）例真正的主语恢复它原来的地位时，便成为：To tell a lie is wrong.这正是中国话的表现法，但英文除极少数的文句外（如To teach is to learn.To eat is to live.）一般都不喜欢用不定词作主语，而要改用形式上的“if”来作主语，把它说成：It is wrong to tell alie.英文用不定词作主语虽不流行，但至少文句是通的。用不定词作宾语，则有背英文的惯用法，成为不通的了。如说成，He does not think to tell a lie wrong.或说成，He does not think wrong to tell a lie.都要不得，必须加用形式上的宾语“it”进去，而把它说成：He does not think it wrong to tell a lie.才合乎英文惯用的语法。因为惯用法不允许拿不定词作宾语，所以凡遇到不定词要用作宾语时，就采用形式上的“it”来代替它。
不定词虽不能用作宾语，但可用作补语，如 We found him to be alive.（我们发觉他还活着。）这种作补语用的不定词，有时可以略去，单说We found him alive.意思也是一样，而且更加简明有力。
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形式上的宾语，在某些常说的文句中，可以省略不用，尤其是后接不定词的时候为然，如：Your father thought（it）fit to leave me guardian.（你父亲认为请我做监护人最为适合。）IfGod sees（it）fit that I should marry，he would provide me with a worthy husband.（如果上帝认为我应该结婚，他就会供给我一个如意郎君的。）
And are you sure of all this，are you sure（of it）that nothing ill has befallen my boy？（Goldsmith）（这一切都是真的吗，你确实知道我儿子没有遭遇任何不顺利的事吗？）I am sorry（for it）that you are going away.（你要走我很难过。）Men differ from brutes in（it）that they can think and speak.（人之所以异于禽兽，就在能思想能说话。）He forgot everything but（it）that he was near her.（Galsworthy）（他跟她在一块儿就把一切都忘了。）
【类例】
It is easy to learn English，but it is difficult to attain perfection in it（英文易学难精。）It is much easier to read French than read Greek.（学法文比学希腊文容易多了。）It may be advisable to wait till they come back.（最好是等他们转来。）She had said what it was necessary to say.（要说的她都说了。）It needs no great sagacity to see that the writer is a novice at his task.（很容易看得出来那作家是一位生手，刚从事写作的。）
I found it very diflicult to put it in practice.（我觉得实行很难。）He took it upon himself to pay off the debt.（他自愿负责还清债务。）Iknow it to be the established custom of your sex to reject a man on the first application.（Jane Austen）（我知道拒绝男子的第一次求婚是你们女性的固定的习惯。）Fortunately I have it in my power to introduce you to very superior society.（幸而我还有能力可以介绍你给非常上流的社会。）If you take it into your head to go on refusing every offer of marriage in this way，you will never get a husband at all.（如果你执意要继续这样来拒绝所有的求婚，你就永远得不到一个丈夫了。）Make it the first object to be able to fix and hold your attention upon your studies.（把专心致志于学业，持久不懈，作为第一目标。）The fog made it difficult to calculate the distance.（雾迷了眼，看不出距离来。）
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